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Dynamic bearing loads

Calculating dynamic bearing loads

The loads acting on a bearing can be calculated according to the laws of mechanics if the external forces, such as forces from power transmission, work forces or inertial forces, are known or can be calculated. When calculating the load components for a single bearing, the shaft is considered as a beam resting on rigid, moment-free supports for the sake of simplification. Elastic deformations in the bearing, the housing or the machine frame are not considered, nor are the moments produced in the bearing as a result of shaft deflection.

These simplifications are necessary if a bearing arrangement is to be calculated without a computer program. The standardized methods for calculating basic load ratings and equivalent bearing loads are based on similar assumptions.

It is possible to calculate bearing loads based on the theory of elasticity, without making the above assumptions, but this requires the use of complex computer programs. In these programs, the bearings, shaft and housing are considered as resilient components of a system.

If external forces and loads like inertial forces or loads resulting from the weight of a shaft and its components are not known, they can be calculated. However, when determining work forces and loads, e.g. rolling forces, moment loads, unbalanced loads and shock loads, it may be necessary to rely on estimates based on experience with similar machines or bearing arrangements.

Geared transmissions

With geared transmissions, the theoretical tooth forces can be calculated from the power transmitted and the design characteristics of the gear teeth. However, there are additional dynamic forces, produced either by the gear, or by the input or output shaft. Additional dynamic forces from gears can be the result of form errors of the teeth and from unbalanced rotating components. Because of the requirements for quiet running, gears are made to such a high level of accuracy that these forces are generally negligible, and not considered when making bearing calculations.

Additional forces arising from the type and mode of operation of the machines coupled to the transmission can only be determined when the operating conditions are known. Their influence on the rating lives of the bearings is considered using an “operation” factor that takes shock loads and the efficiency of the gears into account. Values of this factor for different operating conditions can usually be found in information published by the gear manufacturer.

Belt drives

When calculating bearing loads for belt driven applications, “belt pull” must be taken into consideration. Belt pull, which is a circumferential load, depends on the amount of torque being transmitted. The belt pull must be multiplied by a factor, which depends on the type of belt, belt tension and any additional dynamic forces. Belt manufacturers usually publish values. However, should information not be available, the following values can be used:

toothed belts

1,1 to 1,3

V-belts

1,2 to 2,5

plain belts

1,5 to 4,5

The larger values apply when the distance between shafts is short, for heavy or shock-type duty, or where belt tension is high.